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Message-ID: <CAEvNRgEo2UZ63uv0F7Pv8VfeJipyu82b=Rgiz2gnttdRu9aEPQ@mail.gmail.com>
Date: Wed, 28 Jan 2026 09:07:04 -0800
From: Ackerley Tng <ackerleytng@...gle.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>
Cc: cgroups@...r.kernel.org, kvm@...r.kernel.org, linux-doc@...r.kernel.org,
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Subject: Re: [RFC PATCH v1 01/37] KVM: guest_memfd: Introduce per-gmem
attributes, use to guard user mappings
Binbin Wu <binbin.wu@...ux.intel.com> writes:
> On 10/18/2025 4:11 AM, Ackerley Tng wrote:
> [...]
>>
>> +static int kvm_gmem_init_inode(struct inode *inode, loff_t size, u64 flags)
>> +{
>> + struct gmem_inode *gi = GMEM_I(inode);
>> + MA_STATE(mas, &gi->attributes, 0, (size >> PAGE_SHIFT) - 1);
>> + u64 attrs;
>> + int r;
>> +
>> + inode->i_op = &kvm_gmem_iops;
>> + inode->i_mapping->a_ops = &kvm_gmem_aops;
>> + inode->i_mode |= S_IFREG;
>> + inode->i_size = size;
>> + mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
>> + mapping_set_inaccessible(inode->i_mapping);
>> + /* Unmovable mappings are supposed to be marked unevictable as well. */
> AS_UNMOVABLE has been removed and got merged into AS_INACCESSIBLE, not sure if
> it's better to use "Inaccessible" instead of "Unmovable"
>
Thanks, will update comment as follows:
/*
* guest_memfd memory is not migratable or swappable - set
* inaccessible to gate off both.
*/
mapping_set_inaccessible(inode->i_mapping);
WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
>> + WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
>> +
>> + gi->flags = flags;
>> +
>> + mt_set_external_lock(&gi->attributes,
>> + &inode->i_mapping->invalidate_lock);
>> +
>> + /*
>> + * Store default attributes for the entire gmem instance. Ensuring every
>> + * index is represented in the maple tree at all times simplifies the
>> + * conversion and merging logic.
>> + */
>> + attrs = gi->flags & GUEST_MEMFD_FLAG_INIT_SHARED ? 0 : KVM_MEMORY_ATTRIBUTE_PRIVATE;
>> +
>> + /*
>> + * Acquire the invalidation lock purely to make lockdep happy. There
>> + * should be no races at this time since the inode hasn't yet been fully
>> + * created.
>> + */
>> + filemap_invalidate_lock(inode->i_mapping);
>> + r = mas_store_gfp(&mas, xa_mk_value(attrs), GFP_KERNEL);
>> + filemap_invalidate_unlock(inode->i_mapping);
>> +
>> + return r;
>> +}
>> +
> [...]
>> @@ -925,13 +986,39 @@ static struct inode *kvm_gmem_alloc_inode(struct super_block *sb)
>>
>> mpol_shared_policy_init(&gi->policy, NULL);
>>
>> + /*
>> + * Memory attributes are protected the filemap invalidation lock, but
> ^
> protected by
Thanks!
>> + * the lock structure isn't available at this time. Immediately mark
>> + * maple tree as using external locking so that accessing the tree
>> + * before its fully initialized results in NULL pointer dereferences
>> + * and not more subtle bugs.
>> + */
>> + mt_init_flags(&gi->attributes, MT_FLAGS_LOCK_EXTERN);
>> +
>> gi->flags = 0;
>> return &gi->vfs_inode;
>> }
>>
>> static void kvm_gmem_destroy_inode(struct inode *inode)
>> {
>> - mpol_free_shared_policy(&GMEM_I(inode)->policy);
>> + struct gmem_inode *gi = GMEM_I(inode);
>> +
>> + mpol_free_shared_policy(&gi->policy);
>> +
>> + /*
>> + * Note! Checking for an empty tree is functionally necessary to avoid
>> + * explosions if the tree hasn't been initialized, i.e. if the inode is
>
> It makes sense to skip __mt_destroy() when mtree is empty.
> But what explosions it could trigger if mtree is empty?
> It seems __mt_destroy() can handle the case if the external lock is not set.
>
>
Hope this updated comment clarify the explosion:
/*
* Note! Checking for an empty tree is functionally necessary
* to avoid explosions if the tree hasn't been fully
* initialized, i.e. if the inode is being destroyed before
* guest_memfd can set the external lock, lockdep would find
* that the tree's internal ma_lock was not held.
*/
>> + * being destroyed before guest_memfd can set the external lock.
>> + */
>> + if (!mtree_empty(&gi->attributes)) {
>> + /*
>> + * Acquire the invalidation lock purely to make lockdep happy,
>> + * the inode is unreachable at this point.
>> + */
>> + filemap_invalidate_lock(inode->i_mapping);
>> + __mt_destroy(&gi->attributes);
>> + filemap_invalidate_unlock(inode->i_mapping);
>> + }
>> }
>>
>> static void kvm_gmem_free_inode(struct inode *inode)
>> --
>> 2.51.0.858.gf9c4a03a3a-goog
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